1. Field of the Invention
The present invention generally relates to a mobile communication system. More particularly, the present invention relates to a method for efficiently broadcasting system information in a cell and a method for receiving the system information in a User Equipment (UE).
2. Description of the Related Art
The Universal Mobile Telecommunications System (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in Wideband Code Division Multiple Access (WCDMA), based on European mobile communication systems, Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS). The 3rd Generation Partnership Project (3GPP) that standardized UMTS is now discussing Long Term Evolution (LTE) as the next generation of UMTS, known as Evolved UMTS. The 3GPP LTE is a technology for enabling packet communications at or above 100 Mbps, aiming at commercialization by 2010. For deploying the LTE system, many communication schemes have been proposed. Among them are schemes of reducing the number of nodes on a communication line by simplifying a network configuration or of optimizing radio protocols for radio channels.
FIG. 1 is a diagram illustrating an Evolved UMTS system to which the present invention is applied.
Referring to FIG. 1, each of Evolved UMTS Radio Access Networks (E-UTRANs or E-RANs) 110 is simplified to a 2-node structure including Evolved Node Bs (ENBs) 120 and 122 and an anchor node 130, or ENBs 124, 126 and 128 and an anchor node 132. A User Equipment (UE) 101 is connected to an Internet Protocol (IP) network 114 via the E-UTRAN 110. The ENBs 120 to 128 correspond to legacy Node Bs in the UMTS system and are connected to the UE 101 via radio channels. Compared to the legacy Node Bs, the ENBs 120 to 128 play a more complex role. Since all user traffic including real-time service such as Voice Over IP (VoIP) is serviced on shared channels in the 3GPP LTE, an entity for collecting the status information of UEs and scheduling them is required and the ENBs 120 to 128 are responsible for the scheduling. Generally, an ENB controls a plurality of cells. Generally, the ENBs 120 to 128 perform Adaptive Modulation and Coding (AMC) by adaptively selecting a modulation scheme and a channel coding rate for a UE according to the channel status of the UE. As with High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA) of UMTS (also referred to as Enhanced Dedicated CHannel (EDCH)), the LTE system uses Hybrid Automatic Repeat reQuest (HARQ) between the ENBs 120 to 128 and the UE 101. Considering that a variety of Quality of Service (QoS) requirements cannot be fulfilled with HARQ alone, a high layer may perform an outer ARQ between the UE 101 and the ENBs 120 to 128. HARQ is a technique for increasing reception success rate by soft-combining previous received data with retransmitted data without discarding the previous data. High-speed packet communication systems such as HSDPA and EDCH use HARQ to increase transmission efficiency. To realize a data rate of up to 100 Mbps, it is expected that the LTE system will adopt Orthogonal Frequency Division Multiplexing (OFDM) in a 20-MHz bandwidth as a radio access technology.
FIG. 2 illustrates system information broadcast in cells.
Referring to FIG. 2, reference numeral 201 denotes an ENB and reference numerals 211, 213 and 215 denote transmissions of system information from first, second and third cells, CELL #1, CELL #2 and CELL #3, respectively. The system information includes essential physical parameters and high-layer parameters common to UEs in a cell so that the UEs can receive a service in the cell. The physical parameters include, but are not limited to, the bandwidth of the cell, a Cyclic Prefix (CP) length, a physical channel configuration, the number of transmit antennas, and a System Frame Number (SFN), for example. The high-layer parameters may include a measurement Identifier (ID) and scheduling information about frequency or time resources in which other high-layer parameters are transmitted. A Primary Broadcast CHannel (P-BCH) carries the system information. To stably reach a cell boundary, the P-BCH needs a high transmit power or a robust Modulation and Coding Scheme (MCS) level.
FIG. 3 illustrates an exemplary method for transmitting system information.
Referring to FIG. 3, a 10-ms radio frame 301 includes ten subframes 303. It is assumed herein that a P-BCH carries system information in a 1.25-MHz subframe in every radio frame. As described before with reference to FIG. 2, to stably reach a cell boundary, the P-BCH carries a limited number of bits in a subframe. For example, a very low coding rate is applied to the P-BCH to achieve a 1% BLock Error Rate (BLER) for 98% of the cell coverage area and the P-BCH may deliver no more than 20 to 30 bits of information in a 1-ms subframe with a 1.25 MHz of bandwidth. The limitation on the number of P-BCH information bits makes it impossible to transmit all of the necessary system information on the P-BCH. If system information is ever managed to fit the allowed number of information bits, the system information size cannot be extended for the next transmission on the P-BCH. Accordingly, there is a need for a method for transmitting more information bits on the P-BCH in a given bandwidth.